This invention relates generally to lightwave communication networks and, more particularly, to add/drop arrangements for adding and dropping optical signals in lightwave communication systems.
As is well known, wavelength division multiplexing (WDM) increases transmission capacity of lightwave communication systems. In particular, WDM combines many optical channels of different wavelengths for simultaneous transmission as a composite optical signal in a single optical fiber. At present, WDM has found widespread use in long-haul network applications and is now being contemplated for other applications such as metropolitan optical networks and the like.
In general, the capability to drop, add, or otherwise replace selected optical channels at multiple add/drop nodes in a WDM-based network is essential for a variety of value-added communication services including local access, interactive multimedia, wavelength leasing, and so on. Add/drop capability is especially important in metropolitan optical networks which are typically more densely populated with users in a more geographically limited area as compared to long-haul networks. As an example, metropolitan optical networks may include multiple optical rings each including multiple nodes at which traffic must be added and dropped. In addition to servicing the extensive add/drop requirements, management of bandwidth in these networks can be further complicated by such factors as flow of traffic, diversity of traffic types, and dynamic changes in traffic levels associated with adding and dropping of traffic at the various nodes. Consequently, flexibility in managing bandwidth and costs associated with supporting dynamically changing add/drop requirements have become significant issues in this environment.
Many different types of add/drop arrangements for WDM systems and networks are known. For example, in one type of add/drop system, a complete demultiplexing approach is used whereby an incoming WDM signal is demultiplexed at an add/drop node into individual optical channels of different wavelengths, one or more of the demultiplexed optical channels are then dropped, and the remaining optical channels along with any optical channels to be added are then multiplexed back into a composite WDM signal. A system of this type is disclosed in U.S. Pat. No. 5,526,153 entitled xe2x80x9cOptical Channel Adding/Dropping Filterxe2x80x9d issued on Jun. 11, 1996. In addition to the prohibitive cost associated with placing this type of demultiplexer/multiplexer arrangement at each add/drop node in a metropolitan optical network, for example, this arrangement also has other disadvantages such as spectral narrowing and signal power loss. In particular, spectral narrowing of the optical channel bandwidths can occur as the WDM signal is repeatedly demultiplexed and multiplexed through the various nodes. Moreover, signal power losses across each demultiplexer/multiplexer pair can be substantial, requiring insertion of costly optical amplifiers throughout the network. Additionally, all optical channels are demultiplexed/multiplexed at each node regardless of the number of optical channels actually being dropped and added.
Spectral narrowing can be reduced by employing fiber grating-based selective filters that avoid complete demultiplexing/multiplexing of the WDM signal (see, e.g., Okayama et al., xe2x80x9cDynamic wavelength selective add/drop node comprising fibre gratings and optical switches,xe2x80x9d Electronics Letters, Vol. 33, No. 5, pp. 403-404 (1997), U.S. Pat. No. 5,712,932 entitled xe2x80x9cDynamically Reconfigurable WDM Optical Communication Systems With Optical Routing Systemsxe2x80x9d issued Jan. 27, 1998, and U.S. patent application Ser. No. 08/879,929 entitled xe2x80x9cA Reconfigurable Add-Drop Multiplexer for Optical Communications Systems,xe2x80x9d filed on Jun. 20, 1997. However, these type of add/drop arrangements are limited in terms of their scalability for accommodating new or otherwise dynamically changing add/drop requirements in networks such as metropolitan optical networks and the like. For example, upgrading these prior arrangements to accommodate add/drop requirements which were not contemplated in initial system design (e.g., to add/drop optical channels in a new wavelength band) would result in disruptions to existing service. More specifically, disruptions would occur when inserting the additional fiber gratings and switches in the existing transmission path to service the new add/drop requirements.
A scalable, reconfigurable, and cost-effective add/drop capability is provided according to the principles of the invention using a novel add/drop architecture which enables a wavelength division multiplexed (WDM) system to be upgraded to accommodate new and changing add/drop requirements without disruption in existing service. The add/drop arrangement according to the principles of the invention is based on a modular architecture wherein one or more modular optical routing devices are coupled in an optical fiber path to facilitate the selective adding/dropping of individual optical channels and to facilitate in-service upgrades without disrupting existing transmissions in the optical fiber path.
In an exemplary embodiment, at least two directional optical transfer devices, such as optical circulators, are coupled to a drop path for dropping optical channels from the WDM signal, an add path for receiving optical channels to be added to the WDM signal, and a common path between the directional optical transfer devices. In the common path, at least one optical routing module is employed which is capable of receiving the WDM signal at an input and routing or otherwise switching the WDM signal to outputs, each of which can be coupled to one or more wavlength-selective modules. Each wavelength-selective module can be configured with an arrangement of optical filters, such as fiber gratings, and optical switches to facilitate the selective adding and dropping of one or more individual optical channels. Importantly, the optical routing module includes additional outputs for coupling to additional optical routing modules, additional wavelength-selective modules, and other components in order to accommodate changing add/drop requirements, which is an anticipated characteristic especially of metropolitan optical networks. Because the additional add/drop requirements are serviced by adding components to available outputs of the optical routing modules instead of directly in the optical fiber path which is supporting existing service, in-service upgrades can be performed on the WDM system in a substantially non-disruptive manner.
By using wavelength-selective modules consisting of switches and optical filters, e.g., fiber gratings, the add/drop arrangement is reconfigurable in that one or more optical channels can be selectively added/dropped at a node. Moreover, the scalability problems of prior arrangements are overcome by using optical routing modules to accommodate additional add/drop requirements without disrupting existing service.